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Chow Groups of Projective Varieties of Very Small Degree

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Chow Groups of Projective Varieties of Very Small Degree Vol. 87, No. 1 DUKE MATHEMATICAL JOURNAL (C) 1997 CHOW GROUPS OF PROJECTIVE VARIETIES OF VERY SMALL DEGREE HILINE ESNAULT, MARC LEVINE, AND ECKART VIEHWEG Let k be a field. For a closed subset X of IP,, defined by r equations of degree d > > dr, one has the numerical invariant i=2 dl where [] denotes the integral part of a rational number . If k is the finite field lFq, the number of k-rational points verifies the congruence # lpn(lFq) =- #X(IFq) mod qr, while, if k is the field of complex numbers E, one has the Hodge-type relation n F Hc(lPc X) Hc(IPc X) for all/ (see [12], [5] and the references given there). These facts, together with various conjectures on the cohomology and Chow groups of algebraic varieties, suggest that the Chow groups of X might satisfy CH(X) (R) CH(lP) (R) for < x- 1 (compare with Remark 5.6 and Corollary 5.7). This is explicitly formulated by V. Srinivas and K. Paranjape in [16, Con- jecture 1.8]; the chain of reasoning goes roughly as follows. Suppose X is smooth. One expects a good filtration 0 Fj+l = Fj = = F CW(X X) (R) whose graded pieces FI/Fl+l are controlled by n2j-l(x X) (see [10]). According to Grothendieck's generalized conjecture [8], the groups H(X) should be gen- erated by the image under the Gysin morphism of the homology of a codimension- x subset, together with the classes coming from lP n. Applying this to the diagonal in X x X should then force the triviality of the Chow groups in the desired range. Received 6 November 1995. Revision received 14 May 1996. Authors' work supported by the DFG-Forschergruppe "Adthmetik und Geometric" (Essen). 29 30 ESNAULT, LEVINE, AND VIEHWEG For zero-cycles, the conjecture (.) follows from Roitman's theorem (see [17] and [18]): CHo(X)=Z if di<n. (**) i=l In [16], K. Paranjape proves a version of (.), showing that there is a finite bound N N(dl,... ,dr; l), such that, for n > N, one has CHI,(X) 7z for 0 < l' < I. The bound N(dl,..., dr; l) grows quite rapidly as a function of the degrees; for example, if r 1, one has the inductive inequality l d N(d, 1) + N(d- l, 1) N(d- 1, 1), > ( d- 1 ) + and N(2, 1) is at least five. In this article, we give the following improved bound (see Theorem 4.5). Sup- posedly> >dr>2, andeitherdl>3orr>l+l. If n then CHe(X)(R)= forO<l'<l. If dl dr 2 and r < 1, we have the same conclusion, assuming the modi- fied inequality =r(l+2) n-l+r-1 "(l+di)/+1 < i= As an application, if we assume in addition to the above inequalities that X is smooth, we show in Section 5 that the primitive cohomology of X is generated by the image of the homology of a codimension-(/+ 1) subset, in accordance with Grothendieck's conjecture, and we show that #1pn(IFq)= #X(IFq)mod ql+l for almost all primes p and X defined in characteristic zero. The method of proof of the improved bound is a generalization of Roitman's technique, coupled with a generalization of Roitman's theorem (**) to closed subsets of Grassmannians defined by the vanishing of sections of Syma of the tautological quotient bundle. This latter result is an elementary consequence of the theorem due to Kollir-Miyaoka-Mori [14] and Campana [3] that Fano varieties are rationally connected. The first part of the argument, the application of Roitman's technique to cycles of higher dimension, is completely geometric. CHOW GROUPS OF PROJECTIVE VARIETIES 31 As an illustration, consider the case of surfaces on a sufficiently general hyper- surface X of degree d > 3 in IPn. Roitman shows that, if d < n and p is a general point of a general X, there is a line L in IPn such that L. X dp. Now take a surface Y on X, in a general position. Applying Roitman's con- struction to the general point y of Y, and specializing y over Y, we construct a three-dimensional cycle S in IPn with the property that s.x= J where N is some positive integer and the Y are ruled surfaces in X. If (dl) < n- 1, we can find for each general line L on X a plane II in lPn such that II. X dL. Assuming that the general line in each Y is in general position, we construct a three-dimensional cycle S in IPn such that s' x Nt E njY + Em,l-li where N' is a positive integer, and the Hi are two-planes in X. From this (ignoring the general position assumptions) it follows that all two-dimensional cycles on X are rationally equivalent to a sum of two-planes in X. We may then apply our result on zero-cycles of subsets of Grassmannians, which in this case implies that all the two-planes in X are rationally equivalent, assuming (d2) n. Putting this together gives CH2(X) (R) <n. @ ifd>3and(d+2)3 One needs to refine this argument to treat cases of special position, as well as larger and r. For the reader's convenience, we first give the argument in the case of hypersurfaces before giving the proof in general; the argument in the general case does not rely on that for hypersurfaces. Throughout this article, we assume that k is algebraically closed, as the kernel of CHl(Xk) CH/(X) is torsion (see [1]). Marc Levine would like to thank the Deutsche Forschungsgemeinschaft for their support and the Universitit Essen for their hospitality. 1. Flag and incidence varieties. Let X _ IP be a closed reduced subscheme. For 0 < s < n 1, let tl3rk(s) tBrk(s; n) denote the Grassmann variety of s-planes 32 ESNAULT, LEVINE, AND VIEHWEG in ]P,, and let A(s) -= (13rk(s) x (rk(s) be the universal family. We write r(s;X) for the closed subscheme of 03rk(s) parametrizing s-planes in IP which are contained in X. Correspondingly ?: A(s; X) 3rk(s; X) denotes the restriction of Ys to A(s; X) yj-l(a3rk(s; X)). In rk(s; X) ll3rk(s + 1), we consider the flag manifold lF(s,s + 1;X) consisting of pairs [H, H'] with and H c H' IP+1 _ IP. The x - induces a projection Brk(s; X) Brk(s + 1) Grk(s; X) morphism lF(s,s + 1;X) --* rk(s + 1) and a surjection IF(s, s + 1; X) Gr(s; X). By abuse of notation, we write A(s; X) and A(s + 1) for the pullback of the uni- versal families to IF(s, s + 1;X) and IF(s,s + 1;X) x X A(s;X) A(s + 1) IF(s,s + 1;X) x IP IF(s, s + 1; X) IF(s, s + 1; X) for the induced morphisms. Assume that X _ IP is a hypersurface of degree d. Hence X is the zero set of f (xo, X Xn) k[xo, Xn ]a" We consider the incidence varieties H'= IH'(s,s+ 1;X)=rk(s;X) x tHrk(s+ 1;X) oIF(s,s+ 1;X) and ]H Ill(s, s + 1; X) {[H, H'] IF(s,s + 1; X); H' _ X or H' c X H}. Here "H' c X" denotes the set-theoretic intersection. "H' c X H" implies that the zero-cycle of fin, is H with multiplicity d. We will see in the proof of the fol- lowing lemma that IH _ IF(s, s + 1; X) is a closed subscheme. CHOW GROUPS OF PROJECTIVE VARIETIES 33 By definition, one has IH' _ IH. It might happen that for all [H,H'] e IH the (s + 1)-plane H' is contained in X, or in different terms, that IH'= IH, but for a general hypersurface X, both are different. Generalizing Roitman's construction for s 0 in 17], one obtains the following. LEMMA 1.1. Let X _ P' be a hypersurface of degree d, and let 711" In rk(s; X) and n'" IH' --. Gr(s; X) be the restrictions of the projection prl trk(s; X) x tFJrk(s + 1) -- {Brk(s; X). Then for all [H] e rk(s;X), the fibres of 7[1 (or 7[) are subschemes of aklDn-s-1 defined by equations. In particular, 7[ (or 7[ ) is surjective if n-s or n-s-1 +1 < +1 < --, lln-s-1 Proof. The first projection Pl" IF IF(s, s + 1; X) Grk(s; X) is a 'k bundle (see, for example, [9, 11.40]). In fact, for Spec(A) __. rk(s; X), let us fix coordinates in IP such that AA ?-l(Spec(A))c IP is the linear subspace defined by Xs+I Xs+2 Xn O. Let F be the (n- s)-plane given by Xl Xs 0. An (s + 1)-plane A containing AA is uniquely determined by the line A c F _ F, and each line in F which contains (1 0 0) determines some A. In other terms, there is a Spec(A)-isomorphism -s-1 0"" ]PI pi-l(Spec(A)) _ IF, given by a((ao an-s-l)) [AA, A], where A is spanned by AA and by (1 0 -O: ao an-s-l). 34 ESNAULT, LEVINE, AND VIEHWEG An isomorphism IP4+1 A is given by (0 s+l) (0 s" a0s+l an-s-ls+l).
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